This invention relates to a drive system for a wire dot head in an impact printer. 2) Description of the Related Art
Wire dot printers have attracted a high demand to date because they do not impose any limitation on the type of printing medium and permit printing even on copying paper sheets or the like. Each wire dot printer is provided with a wire dot head which is constructed to drive each print wire by magnetic attractive force of an associated permanent magnet.
Such wire dot printers can each be classified into one of three groups depending on the type of its wire dot head, namely, into the plunger type, the spring-charged type or the clapper type.
In a spring-charged printer out of these printers, each armature with a print wire secured thereon is supported for rocking motion on a biasing leaf spring so that the armature is attracted on a core by a permanent magnet against resilient force of the biasing leaf spring. Upon printing, a coil wound around the core is energized to produce a magnetic flux in a direction opposite to the magnetic flux of the permanent magnet, whereby the armature is released from the core.
Incidentally, the drive time of a wire dot head has heretofore been determined in a whole-sale manner by a time circuit to set the drive time at an optimal value predicted based on empirical data of voltage variations of a power supply.
Due to variations in the characteristics of the wire dot head, variations in the distance between a free end of each print wire and a printing medium, magnetic interference between coils, variations in the characteristics of the coils, etc., the drive time set by the timer circuit may, however, deviate from a drive time actually required.
A shorter drive time leads to the problem that the impacting speed of the free end of each print wire against the printing medium is small and the energy upon printing is hence small to result in poor printing quality or the free end of each print wire does not reach the printing medium to make it impossible to perform printing. A longer drive time, on the other hand, leads to a delay in the application of attractive force subsequent to the projection of each print wire, so that the print wire is delayed in returning to its home position. The print wire cannot therefore project in time for the next print so that printing must be performed at a lower printing speed.
Wire dot head drive systems have therefore been provided, which can maintain the printing quality without the need for lowering the printing speed even when the required drive time varies from one printing operation to another or from one print wire to another (U.S. Pat. No. 4,940,343 and U.S. Pat. No. 5,030,020).
In each of such wire dot head drive systems, each print wire or its associated armature is provided with a sensor for detecting each displacement of the print wire or armature. A signal corresponding to the displacement of the print wire is obtained from the sensor to extract the initiation timing of a movement of the print wire and the timing of an impact of the printing wire against the printing medium. At these timings, an electric current to be fed to a corresponding coil is controlled.
In each of the conventional wire dot head drive system described above, however, the sensor provided for the detection of each displacement of each print wire or armature tends to be affected by noise because it relies upon the measurement of a resulting minute variation in capacitance. A large variation in the voltage of a power supply, which takes place by an induction noise from an adjacent circuit pattern on a printed circuit board or upon driving another print wire, may not be eliminated by a low-pass filter incorporated in a sensor circuit so that a timing pulse extractor may extract such a large voltage variation as a timing signal. In this case, signals may be switched over at a timing earlier than that expected initially. Further, a rebound of a print wire operated immediately before a sensor signal may change over the sensor signal at a timing much earlier than that expected initially.
In such a case, the time of impression of a drive voltage to a coil becomes shorter so that a printing blur or a missing dot tends to occur. Enhancement of a power supply pattern or the provision of a shield to avoid influence of noise, however, results in an increase in the number of parts, thereby leading to a larger and more costly wire dot head.
If the performance of a wire dot head is deteriorated by difficulties in feeding an electric current to each coil due to poor contact of connectors arranged in a current flow path or an unduly large drop in the voltage of a power supply or by an excessive reduction in the resilient force for each print wire, sensor signals are switched over at a timing much slower than that expected initially. In addition, no sensor signal may be outputted in some instances for deficient sensors, improper setting of the voltage, an excessive head gap or difficult movements of print wires. Even in such a case, the drive voltage is still impressed continuously. This leads not only to an increase in the power consumption by the coils and drive circuits but also to a malfunction of the wire dot head due to the generation of heat, burning, breakage of print wires, or the like.
Further, an unduly wide head gap results in an excessive delay in the timing at which print wires impact a printing medium. In this case, the return of each print wire subsequent to an impact is delayed so that the print wire may not be able to assume its home position in time for the next printing or the print wire may catch an ink ribbon to cause printing smear or print wire breakage. When a print wire catches the ink ribbon or printing medium or is broken for other reasons, the timing at which print wires impact the printing medium is also delayed excessively.